The present invention relates generally to tensile test and fatigue measuring devices and particularly to a test system for measuring Lorentz force tensile stress5 strain and critical current density fatigue characteristics of conductors in situ.
The stress, strain, and fatigue characteristics of conductive materials determine the applications of such materials. For example, in conductors such as Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.x /Ag (BSCCO/Ag) and Ag-alloy composite superconductors that have a relatively high critical current density, J.sub.c, large strains can cause irreversible damage to the conductors. Likewise, crack propagation from fatigue at low strains can limit the uses of the conductors. These forces can also affect the superconducting properties of the conductors. For example, the local peak strain within the superconductor filaments of a composite ceramic superconductor can limit its coil properties during high field operation in a superconducting magnet. Further, cycling at low strain may degrade the critical current density and, thus, the superconducting properties, of superconducting tapes. Such fatigue behavior may relate to the fundamental J.sub.c limit and limit other applications of these superconductors as well.
The conventional tensile test system determines the stress-strain relationship of a conductor by mechanically pulling the conductor and comparing the applied force to the amount that the conductor is deformed. Due to the small size of a typical BSCCO/AgX superconducting tape, standard testing machines (i.e., hydraulic and pneumatic MTS type apparatus) are unusable for determining the stress, strain and fatigue characteristics of such tapes. Further, the loads required to fail such tapes usually do not exceed 200 N (45 lbf) which is less than the typical minimum load of conventional test apparatus. Also, these superconducting tapes are often very thin (e.g., having a width to thickness ratio on the order of 20 or more) which necessitates special gripping techniques by the conventional test apparatus. Thus, smaller and more sensitive instrumentation is desired to perform mechanical stress testing.
One such test apparatus is a table-top mechanical testing device operating at room temperature and employing a linear stepper motor capable of delivering 330 N (75 lbf) to a sample. This device uses relatively small 50 and 150 lbf load cells and linear variable displacement transducers (LVDT) for measurement of applied load and displacement. The typical load cell has a resolution of 6.0 mN and the typical LVDT has a resolution of 0.6 .mu.m. In operation, the sample to be tested is placed between a set of grips and pulled. The device accommodates samples of 30 to 50 mm, for example, and runs the samples through a tensile test until failure or a fatigue test using either a strain limit or a test limit.
Disadvantageously, conventional test apparatus fail to provide an indication of stress and fatigue behavior at normal operating temperatures and subject to the actual forces which may eventually cause failure (i.e., magnetic stresses due to Lorentz forces in a magnet). For this reason, an in situ test system which approximates normal operating conditions is desired.